The almost universally used process for the manufacture of alumina is the Bayer process. In a typical commercial Bayer process, the Bayer process stream begins with the pulverization of raw bauxite to a finely divided state. The pulverized ore is then fed to a slurry mixer where a 50% solids slurry is prepared using spent liquor. This bauxite slurry is then diluted with a highly alkaline sodium hydroxide solution and then sent through a series of digesters at temperatures of about 1400-300.degree. C. and under pressures of about 50-1500 p.s.i. Under these conditions, typically 98% of the total available alumina is extracted from the ore which may contain both trihydrate and monohydrate forms of alumina. In the next stage, the Bayer process stream that exits from the digesters passes through a series of flash tanks where heat and condensate are recovered as the digested slurry is cooled to about 110.degree. C. and brought to atmospheric pressure. This digested slurry typically contains a solution of sodium aluminate and about 3-8% of insoluble particles ("red mud").
After removal of the coarse solid particles ("sand"), the slurry of sodium aluminate and insoluble particles is fed to the center well of a mud settler. As the insoluble particles settle, partially clarified sodium aluminate solution, referred to as "green" or "pregnant" liquor, overflows a weir at the top of the mud settling tank and this Bayer process stream is then passed to filtration. Filtration is generally necessary because the mud settler only partially separates the red mud from the sodium aluminate solution. The filtered sodium aluminate solution is then passed to the precipitation stage, where it is cooled to precipitate the trihydrate. The settled solids are withdrawn from the bottom of the mud settler and passed through a countercurrent washing circuit for recovery of sodium aluminate and caustic.
The red muds include various components of the Bayer process stream that are insoluble under highly alkaline conditions, including insoluble or colloidal iron. It is important to rapidly and cleanly separate the red mud from the sodium aluminate solution in order to make this particular step economically efficient. If the rate of separation is too slow, output is materially diminished and overall process efficiency impaired. Likewise, if the separation is not clean, the resultant alumina is somewhat crude and contains sufficiently high levels of iron to make it undesirable for a number of end uses.
The iron present in the Bayer process streams may be in the form of various particulate minerals, soluble iron compounds, and/or insoluble colloidal species. A majority of the iron is normally removed by flocculation of the red mud during the Bayer process. However, there remains a problem in that some insoluble iron species i.e. insoluble colloidal iron species are small enough to pass through the filters. Insoluble colloidal iron may be formed during the Bayer process by the precipitation of iron from solution as the digested slurry is cooled. Under the highly alkaline conditions present in the Bayer process stream, i.e. pH greater than 11, usually greater than 12 or even 13, iron may have a solubility of greater than 30 milligrams per liter of Bayer process stream at the high temperatures and pressures existing during digestion, but has much lower solubility at the clarification temperature. For instance, it has been reported that the solubility of iron in NaAlO.sub.2 solution is about 2 milligram per liter at the clarification temperature, see P. Basu, G. A. Nitowski and P. J. The, "Chemical Interactions of Iron Minerals During Bayer Digest and Clarification," in Iron Control in Hydrometallurgy, Eds. J. E. Dutrizac and A. J. Monhemius, Ellis Horwood Limited, 1986, pp. 223-244. When the digested slurry is cooled, the iron often precipitates in the form of fine insoluble particles (&lt;1000 .ANG. diameter) of colloidal iron. Because of their tiny size, these particles settle so slowly that they may pass through the mud settler and also pass through the pores of even a one-micron filter.
The problem of colloidal iron that passes through the filtration stage is serious because the iron remains in the sodium aluminate entering the precipitation step and thus contaminates the alumina recovered as the main product of the Bayer process with unacceptable levels of iron. Existing techniques have not completely and adequately solved the iron removal problem. U.S. Pat. No. 4,767,540 discloses the use of polymers containing hydroxamate groups, which give improved settling of the fine muds, resulting in overflow liquors with improved clarities and reduced iron content. U.S. Pat. Nos. 3,088,798 and 3,088,799 disclose the use of polyamidoxime to remove soluble metal species from solution at low pH. WO 91/18026 discloses the use of acrylamidoxime/acrylhydroxamic acid polymers as flocculating agents in water treatment. U.S. Pat. No. 4,083,925 discloses the separation of ferrous iron from alkali metal aluminate liquor by contacting it with anionic polyacrylamide under special conditions within the mud settler. U.S. Pat. No. 4,717,550 discloses the use of tertiary hydroxyl-containing polyamines to reduce the iron content of Bayer process streams. All patents, patent applications, and articles mentioned in this application are hereby incorporated herein by reference.
However, there is still a need for a process that efficiently and effectively reduces the amount of insoluble or colloidal iron so as to reduce or avoid contamination of the final alumina product with either iron or the agent added to remove the iron.